The present invention relates to circuit breakers which perform a tripping operation to thereby open contacts of main power line supplying loads with AC power when detecting an fault current, and more particularly to such circuit breakers having a fault current magnitude determining circuit arranged into a digital signal processing circuit.
Japanese Laid-open Patent Application No. 62-173930 discloses one of the above-described type circuit breakers, which is arranged as follows. Main power line contacts are interposed between a three-phase AC power source and main power line conductors for the three phases. The main power line contacts are opened when an automatic tripping device phases are provided with current transformers detecting load currents flowing through the main power line conductors, respectively. Secondary outputs of the current transformers are full rectified and then converted by I/V converters to three kinds of analog voltage signals indicative of the load current values of the three phase, respectively. A maximum phase discriminating circuit is provided at the output side of each I/V converter. Each discriminating circuit selects a maximum signal from among the analog voltage signals for the three phases and supply it to a signal conversion circuit. The signal conversion circuit is provided for obtaining an effective or average value of the analog voltage signals inputted. The value obtained by the signal conversion circuit is converted to a corresponding digital voltage signal by an A/D conversion circuit and then supplied to a microcomputer. The microcomputer determines the level of the load current indicated by the digital voltage signal input thereto. Based on the result of the load current level determining operation, the microcomputer performs a predetermined time-limiting operation and thereafter drives the automatic tripping device, thereby opening the main circuit contacts.
In the above-described conventional circuit breaker, the signal conversion circuit as an analog signal processing circuit is employed in order that the effective load current value is obtained. Accordingly, a large number of analog circuit elements need to be arranged, which complicates the circuit arrangement and increases the production cost of the circuit breaker. Furthermore, troublesome works are needed for adjustment of the output level of the signal conversion circuit.